There are hundreds of 1000-year-old underground tunnel systems in Europe, and nobody really knows who built them.


Once in a while archaeologists will come across a tunnel or artifact and have no idea where or when it came from.

Even then, it’s usually possible to figure that out eventually. It’s remarkable, then, that there are hundreds—possibly thousands—of underground tunnels under Europe that are a total mystery.

http://www.omgfacts.com/Interesting/There-are-hundreds-of-1000-year-old-unde/62348